1. Field of the Invention
This invention relates to a method and apparatus for consolidating particulate material, such as powders, and more particularly, to a system and method for consolidating particulate material by applying relatively long duration current flow at relatively low current densities to the particulate material in order to achieve densities in excess of ninety percent (90%) of the theoretical maximum density for the particulate material.
2. Description of Related Art
The consolidation of particulate material under relatively high compaction pressure using molds and dies to manufacture parts has become a frequently used industrial process. One of the major limitations of the powder material compaction process is that, with most materials, less than full densification is achieved during the compaction process. Typically, powder material consolidation results in less than ninety-three percent (93%) of its full theoretical density for many powders and for difficult to compact materials (such as stainless steel) less than eighty-five percent (85%) of theoretical density is achieved. Less than full density, results in degraded material properties, such as strength, stiffness, magnetisity and the like. High density is required to enable particulate material consolidation to make higher performance parts, such as gears, for example, for use in automobiles because high strength is often required.
U.S. Pat. Nos. 4,929,415; 4,975,412; 5,084,088; 5,529,746; 5,380,473 are examples of consolidation techniques of the type used in the past. For example, Okazaki discloses a method for sintering and forming powder. This method uses a high voltage of 3 KV or more which is applied to a mold filled with the powder using an electrode which maintains a high current of 50 KAcmxe2x88x922 or greater for a period of time from 10 to 500 microseconds.
Similarly, U.S. Pat. No. 4,975,412 also discloses a method of processing superconducting materials which utilizes, again, a high voltage and current density to provide sharp bonding between or among the particulate material.
Still another example is U.S. Pat. No. 5,529,746 issued to Knoss which discloses processing the powders using one to three electric current pulses from 5xc3x9710xe2x88x925 to 5xc3x9710xe2x88x922 second duration and high electric power applied to the punches of the press.
Thus, the typical technique for consolidating the particulate material is to use a relatively high current pulse of fairly short duration to cause consolidation of the powder. A problem with this approach has been, that under these conditions electrical arcing may occur at the interface between the powder and the current-conducting punches. This arcing will severely limit the useful life of the punches and, therefore, must be overcome in order to make this technique commercially viable.
Still another problem of the prior art is that the walls of the molds or dies used during the consolidation process required an insulator, such as ceramic. One significant problem with this approach is that the ceramic used for insulating the walls were not suitable for generating parts having shapes which require intricate details because when the intricate details are machined into the ceramic insulators and the insulators in the die, the ceramic would sometimes crack or chip upon use during the consolidation process.
Another problem with prior art techniques is that they did not permit tailoring of the power input to the powder mass to allow controlled power input. This resulted in inconsistent densification of parts manufactured using the consolidation process.
What is needed, therefore, is a system and method for consolidating powders which will avoid the problems encountered by the techniques used in the past.
It is, therefore, a primary object to provide a system and method for using relatively long duration, relatively low current density, proximately constant voltage electrical current flow through the particulate material during the consolidation process.
Another object of the invention is to provide a system and method for consolidating particulate material using relatively long duration, relatively low current density in a manner that will permit achievement of ninety-eight percent (98%) or greater of the material""s theoretical density, even when used with materials which traditionally have been very difficult to consolidate, such as stainless steel, Sendust, 4405 and the like.
Another object of the invention is to provide a system and method for avoiding undesired arcing at the interface between the punch and particulate material, thereby improving the useful life of the punches.
Another object of the invention is to provide a consolidation system and method which may utilize either a DC voltage source or a near constant AC voltage source while the current density is kept below about 10 KA/cm2 and the duration of the current discharge maintained longer than 0.1 second, depending on the powder being consolidated.
Still another object of the invention is to provide a consolidation system and method which realizes only modest temperature rises in the powder during the consolidation process.
Yet another object of the invention is to provide a consolidation system and method which utilizes active feedback control of the power input during the consolidation process, thereby permitting tailoring of the power input to the particulate material being consolidated.
Still another object of the invention is to provide an active feedback control for controlling the power input which facilitates realizing controlled densification.
Yet another object of the invention is to provide a system and method for providing a non-ceramic insulator which facilitates developing intricate molds or dies which have not been realized in the past so that intricate details, such as gear teeth on an outer periphery of a gear may be easily manufactured.
In one aspect, this invention comprises a powder consolidation system comprising a powder die for receiving a powder to be consolidated, a first punch and a second punch which cooperate with the powder die to compress the powder, a power source coupled to the first and second punches to energize the powder to a predetermined energy level when the powder is being consolidated, and a feedback control coupled to the punches and the power source for monitoring a characteristic of the powder when it is being consolidated and generating a feedback signal in response thereto, the power source adjusting the predetermined energy level in response to the feedback signal while the powder is being consolidated such that the powder achieves at least ninety-eight percent (98%) of its maximum theoretical density.
In another aspect, this invention comprises a method for consolidating a powder comprising the steps of situating a powder in a powder die, compressing the powder in the powder die using a first punch and a second punch, energizing the powder to a predetermined energy level during the compressing step, monitoring a characteristic of the powder during the compressing step and generating a feedback signal in response thereto, and adjusting the predetermined energy level in response to the feedback signal during the compressing step.